1. Field of the Invention
The present invention relates to an image heating apparatus suitably used as a heat fixing apparatus mounted on an image forming apparatus such as a copier and laser beam printer.
2. Related Background Art
The conventional electrophotographic type of image forming apparatus uses a heat-roller type of heat-fixing device with a halogen heater as a heat source, or a film-heating heat-fixing device using a ceramic heater as a heat source, as means of thermally fixing a toner image on a recording material.
A temperature detection element such as a thermistor is provided on the heat-fixing device. The temperature detection element detects a temperature of the heat-fixing device to vary an electric current applied to a heater, adjusting the temperature of the heater to a target temperature. The temperature is controlled by using a proportional plus integral (PI) control or a proportional plus integral plus derivative control (PID) control. The power is controlled by using a wave number control. The wave number control is a power control method to control the power supplied to a heater by defining one wave by a half wave of an alternating-current waveform and controlling the wave number applied to the heater out of a predetermined wave number (hereinafter referred to as basic wave number).
FIG. 8 is a timing chart illustrating the case where the temperature is controlled by the PI control to substantially change a set temperature at a time. Reference characters 8a, 8b and 8c denote a set temperature, a supplied power and flicker at this point respectively. If the set temperature of 8a is substantially changed from a temperature A to a temperature B, the electric power supplied to the heater suddenly changes as indicated by 8b. This steeply varies the power supply voltage, which sometimes generates flicker as indicated by 8c. Flicker is a phenomenon in which a voltage is periodically dropped due to the impedance of indoor wiring when current flowing into a load is periodically changed to cause flicker of an incandescent lamp connected to the same indoor wiring to which a load apparatus is also connected. In general, the steeper the variation in power supply voltage, the greater the degree of the flicker.
Japanese Patent Application Laid-Open No. H10-186937 discloses two methods of suppressing flicker which causes a problem when the set temperature is substantially changed from the temperature A to the temperature B. A first method stepwise changes the set temperature of the heater little by little. A second method gradually changes the temperature of the heater while electric power supplied to the heater is limited to a constant for a given length of time.
FIG. 9 is a timing chart in the case where the set temperature is stepwise changed from the temperature A to the temperature B. Reference characters 9a, 9b, 9c and 9d in the figure denote a set temperature, the temperature of the heater, a supplied power and flicker at this point respectively.
FIG. 10 is a timing chart in the case where the supplied power is stepwise changed. Reference characters 10a, 10b and 10c in the figure denote a set temperature, a supplied power and flicker at this point respectively.
The electric power supplied to the heater depends on the difference between the set temperature and the temperature detected by the temperature detection element for detecting the temperature of the heater. For this reason, a waveform of a current flowing into the heater also depends on the difference between the set temperature and the temperature detected by the temperature detection element for detecting the temperature of the heater. As illustrated in FIG. 9, even if the set temperature is constant, the temperature of the heater causes a ripple, so that, even if the set temperature is constant, there is varied a difference between the set temperature and the temperature detected by the temperature detection element for detecting the temperature of the heater. For this reason, if the set temperature is stepwise changed like the first method, and even if the set temperature is within a period of time, the output wave-number within the period is uncertain, so that the waveform of current flowing through the heater is variously changed. Human eyes are most sensitive to flicker of approximately 8.8 Hz. Therefore, the smaller the flicker is than 8.8 Hz or the larger the flicker is than 8.8 Hz, the lower the sensitivity, forming an electrification (current application) pattern producing a variation in voltage around a frequency high in visual sensitivity, depending on the combination of the output wave-numbers, which has sometimes not been very effective in suppressing flicker.
Also in the second method, there are various combinations of change in the output wave-numbers related to variations in the power supply voltage and disturbance, forming the electrification pattern producing variation in voltage around a frequency high in visual sensitivity, depending on the combination of the output wave-numbers, which has sometimes not been very effective in suppressing flicker.
FIG. 3 is a chart illustrating an electrification pattern of each level in the wave number control with a basic wave number of 14 and an output wave number of an 8-step level. The half wave indicated by oblique lines in FIG. 3 represents a voltage to be applied. FIGS. 11A and 11B illustrate examples in which a flicker suppressing effect is varied with a combination of output wave numbers in the wave number control with the output wave number being the electrification pattern illustrated in FIG. 3. Reference characters 11a and 11c represent how output wave numbers are varied. Reference characters 11b and 11d represent flicker in the output waves represented by reference characters 11a and 11b, respectively.
When the output wave number is varied from 8 waves to 0 waves, a combination of the output wave numbers in the case where the output wave numbers of 8, 6, 4, 2 and 0 are sequentially varied (refer to 11a in FIG. 11A) causes a change in voltage whose frequency is higher in visual sensitivity than a combination of the output wave numbers in the case where the output wave numbers of 8, 4 and 0 are sequentially varied (refer to 11c in FIG. 11B). For this reason, the peak value of flicker in the case where the output wave numbers of 8, 4 and 0 are sequentially varied (refer to lid in FIG. 11B) is lower than the peak value of flicker in the case where the output wave numbers of 8, 6, 4, 2 and 0 are sequentially varied (refer to 11b in FIG. 11A) when the output wave numbers are varied from 8 to 0. The pattern of an electric power supplied to the heater in 11a is varied more gently than that in 11c. However, the pattern in 11a is sometimes inferior to that in 11c in the level of flicker.